The invention relates to a process for the preparation of a catalyst suitable for the dehydrogenation of paraffins with two, three or four carbon atoms per molecule.
Olefins with two, three or four carbon atoms per molecule can be converted into C.sub.5 + aromatic hydrocarbon mixtures in high yields at relatively low temperatures by contacting the olefins with a crystalline metal silicate having a special structure as catalyst. Said crystalline metal silicates are characterized in that, after one hour's calcination in air at 500.degree. C., they have the following characteristics:
(a) thermally stable up to a temperature of at least 600.degree. C., PA0 (b) an X-ray powder diffraction pattern in which the strongest lines are the four lines mentioned in Table A. PA0 (c) in the formula which represents the composition of the silicate expressed in moles of the oxides and which, in addition to SiO.sub.2, includes one or more oxides of a trivalent metal A, chosen from the group formed by aluminum, iron, gallium and chromium, the SiO.sub.2 /A.sub.2 O.sub.3 molar ratio is higher than 10. PA0 (a) impregnating an alumina carrier with an aqueous solution of a tin compound; PA0 (b) calcining the composition; PA0 (c) impregnating the composition with an aqueous solution of a platinum compound; PA0 (d) reducing the composition; PA0 (e) removing at least part of the halogen ions, which were introduced into the composition by using a halogen-containing platinum compound and/or tin compound in steps (a) and/or (c), by treating the composition with a non-acidic aqueous solution comprising NH.sub.4 + ions to provide a final catalyst containing less than 0.1%w of halogen; and PA0 (f) impregnating the composition with a non-acidic aqueous solution of an alkali metal compound.
TABLE A ______________________________________ d(.ANG.) ______________________________________ 11.1 .+-. 0.2 10.0 .+-. 0.2 3.84 .+-. 0.07 3.72 .+-. 0.06 ______________________________________
In this patent application a crystalline silicate having a thermal stability of at least 600.degree. C. should be taken to be a silicate whose X-ray powder diffraction pattern remains substantially unchanged upon heating to a temperature of 600.degree. C.
Converting paraffins with two, three or four carbon atoms per molecule (for the sake of brevity hereinafter referred to as "C.sub.2 -C.sub.4 paraffins") in the same way is much harder and requires considerably higher temperatures, resulting in a substantial degree of cracking and low yields of C.sub.5 + hydrocarbons. A solution to this problem may be found in a two-stage process in which the paraffins are partly converted into olefins by dehydrogenation, whereupon in a second stage these olefins are converted over the silicate catalyst. Since the second stage of this process is carried out at a relatively low temperature at which only olefins formed in the first stage are converted and no further dehydrogenation of paraffins occurs, it is an optimum course of the first stage on which the success of the process largely depends. A decisive part is played in this respect by the performance of the catalyst used in the first stage. In addition to the catalyst's activity and selectivity to olefins, its stability is of particularly great importance, since the first stage is carried out at a high temperature and in the presence of olefins, which favors polymerization reactions and coke formation.
The present patent application is based on the results of an investigation into the use as catalysts for the dehydrogenation of C.sub.2 -C.sub.4 paraffins of alkali metal promoted compositions which include the metal combination platinum/tin supported on alumina as a carrier. At first these catalyst were prepared in the conventional manner by impregnation of an alumina carrier by using aqueous solutions of SnCl.sub.2 and H.sub.2 PtCl.sub.6 as impregnation liquors. In view of the highly unsatisfactory results obtained when using these chlorine-containing catalysts for the dehydrogenation of C.sub.2 -C.sub.4 paraffins an evaluation was made of the possibilities of improving on these results by using catalysts which had been prepared by the above-described conventional method, but whose chlorine content had been reduced by a steam treatment to less than 0.1%w in the final catalyst and also by using substantially chlorine-free catalysts by carrying out the preparation using aqueous solutions of sodium stannate and tetramine platinum hydroxide as impregnation liquors. Although these catalysts showed a somewhat better performance in the dehydrogenation of C.sub.2 -C.sub.4 paraffins, their stability was still far too low for use on a technical scale.
It is known to prepare Pt/Sn/Al.sub.2 O.sub.3 dehydrogenation catalysts containing alkali metal or alkaline earth metal promoters by the methods described in U.S. Pat. No. 3,998,900.